Display screen

ABSTRACT

A display screen may be formed with a moth-eye like array of elements of sufficiently small size to reduce glare from ambient light while passing outbound image light substantially unaffected. The screen may be used in direct view and projection displays.

BACKGROUND

This invention relates generally to screens for displays includingscreens over direct view displays and screens upon which an image isprojected in projection displays.

Displays are commonly used to show an image which has beenelectronically generated. Displays are found in televisions receivers,computer monitors and projection display systems, processor-basedappliances, toys and games and in a variety of hand-held devicesincluding personal digital assistants, telephones, and the like. Thesedisplays may generally be divided into direct view displays, in which animage is viewed directly by the user, and projection displays, in whichthe image may be projected onto a larger surface so that a larger imageresults which is more readily viewable by one or more viewers.Projection displays may include those that use reflective techniques andthose that use transmissive techniques for generating the projectedimage. In each case, a screen ultimately provides the image for view bythe user.

Display screens are subject to a number of shortcomings. Commonly, thedisplay screen may produce glare as a result of the reflection ofambient light from flare and ghosting. In addition, displays may besubject to graininess wherein the particles forming the display screenor the imaging device itself create pixel boundaries which are readilyviewable by the user. Some displays are subject to a defect calledpixellation where the graininess of the imaging device is visible to theuser.

In addition, some displays, including those that are using coherentlight sources, may be more susceptible to interference effects resultingin speckle. Speckle is a plurality of point light images which come fromthe display and may move when the user moves with respect to thedisplay. To some degree, speckle artifacts may be enhanced by thegraininess of the imaging device or the display screen.

Some displays may have limited fields of view so that the viewing anglemay be restricted. If the user moves beyond the viewing angle of thedisplay, contrast reversal may result.

It is known to use a diffuser over display screens to improve theviewing angle and to overcome the graininess of the image that isviewed. For example, some display screens use a diffuser in combinationwith a Fresnel lens to overcome these effects. However, such systems dolittle to overcome the effects of ambient light on the display and theglare that may result. In addition, many of the techniques forovercoming graininess and speckle are less than completely effective.

More recently, rear projection television screens have been based on alaminate structure combining lenticular and Fresnel optical elementsthat may achieve increased brightness uniformity, increased imagebrightness in a preferred viewing cone and reduced glare. A Fresnel lensforms an image of the projection lens aperture in the middle of theviewing area which is typically a small region. This small region isenlarged by adding a weak diffuser which expands the viewing area alongthe vertical direction. A lenticular array stretches the viewing areaalong the horizontal dimension. A holographic diffuser can replace aconventional diffuser and a lenticular array.

While existing designs may provide adequate gain, brightness andcontrast, they may not provide adequate ambient light rejection due toback reflection from the screen. In some cases, speckle may be a problemdue to the finite grain size on the diffusing screen, which may be aparticularly severe problem under coherent illumination.

Thus, there is a continuing need for techniques which provide adequategain, brightness and contrast while improving the ambient lightrejection and, in some applications, reducing speckle.

SUMMARY

In accordance with one aspect of the present invention, a display screenincludes a support structure. A moth-eye pattern of light absorbingelements is formed on the support structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction of one embodiment in accordance with thepresent invention;

FIG. 2 is a schematic depiction of a projection display in accordancewith one embodiment of the present invention;

FIG. 3 is a schematic depiction of a direct view display in accordancewith one embodiment of the present invention;

FIG. 4 is a perspective view of the embodiment shown in FIG. 1;

FIG. 5 is an enlarged perspective view of a portion of the displayscreen shown in FIG. 2 showing how ambient light is rejected by thedisplay screen in one embodiment of the present invention; and

FIG. 6 is an enlarged perspective view of moth-eye like element usefulin another embodiment of the present invention.

DETAILED DESCRIPTION

Referring to FIG. 1, a display screen 10, receiving image light to bedisplayed through or on the screen, rejects ambient light coming fromthe viewer's side of the screen 10. In this way, the screen can provideadequate gain, brightness and contrast while improving ambient lightrejection and reducing speckle, at least in cases where coherentillumination is involved. The viewer, indicated in FIG. 1, expects tosee a display with good gain, brightness and contrast. At the same timethe viewer prefers not to have ambient light reflected back as glareinto the viewer's line of sight.

The display screen 10 may be a display screen for a projection viewdisplay 26, as shown in FIG. 2. In such case, a projector 28 projectslight that is viewed on the screen 10. In an embodiment with a rearprojection system, a mirror 30 may be used to reflect an image throughthe screen 10 to a viewer on the outside. In some cases, the image lightmay be formed by a reflective optic system so that the viewer views theimage through the display screen 10. In other cases, a transmissiveoptical system may be utilized for example for a front projectiondisplay.

The display screen 10 may be a direct view display 32 screen, asindicated in FIG. 3, in which case the image light may be formed by animage forming device 34 such as a spatial light modulator, that isrelatively close to the display screen 10 itself.

The screen 10, in one embodiment, includes a Fresnel lens 12 andbroadband holographic phase plate 14, shown in FIG. 1. Which the lens 12and plate 14 are shown in a face to face orientation the lens 12 andplate 14 may be separated as well. The phase plate 14 may perform therole of a diffuser which controls brightness, contrast, gain and theresolution of the screen. In addition, the holographic phase platerejects ambient light. It may provide for both horizontal spread andvertical spread.

As shown in FIG. 4, the holographic phase plate may have a plurality oflight absorbing elements 16 formed on a substrate 18. The substrate 18may include the layers 12 and 14 which form the screen 10. The elements16 may be light transmissive in one embodiment of the present invention.The effect of the light absorbing elements 16, best shown in FIG. 5, isto cause a plurality of labyrinthine surface reflections eitherinternally of the elements 16 as indicated at 22 or between adjacentelements as indicated at 24. By virtue of the moth-eye like structures16, light trapping reduces back reflections or glare.

Since the graininess of the display may be overcome, the effects ofpixellation and limited viewing angle may also be improved with thescreen 10. Reducing graininess may also reduce speckle by increasing theinteraction length (or optical path length) of the screen.

The elements 16 may be fabricated on planarized surfaces using atechnique called holographic lithography. See U.S. Pat. No. 5,142,385,assigned to Massachusetts Institute of Technology; N. P. Economous, etal., “A Simple Technique for Modifying the Profile Reset Exposed byHolographic Lithography,” J. Vac. Sci. Tec. 19, 1234 (1981). HolographicLithography Systems, Inc. of Bedford, Mass. has produced such moth-eyestructures using a maskless holographic technique which allowspatterning by the interference. Feature sizes as small as 90 nanometersmay be formed over large areas. Using this technique, moth-eye likesurface relief array structures may be formed of an array of microscopicprotrusions.

Holographic lithography is a maskless holographic technique which allowsthe patterning, by interference, of microscopic feature sizes.Holographic lithography involves a periodic or quasi periodic patternexposed in a photosensitive film by overlapping two beams from a laseror other coherent source.

In one particular implementation of holographic lithography, termed“achromatic holographic lithography,” gratings are used to split andrecombine the beams (see E. Anderson, K. Komatsu and H. I. Smith,“Achromatic Holographic Lithography in the Deep UV,” J. Vac. Sci, Tech.B6, 216 (1988)). As a result, the source need not have a high degree oftemporal coherence (i.e., narrow bandwidth) or spatial coherencecommonly seen in laser sources. The minimum period, p, (i.e.,center-to-center distance between adjacent lines) obtainable inholographic lithography is given by

p=λ/2 sin θ

where λ is the wavelength of the exposed radiation and θ is half theangle between the intersecting beams. This angle may be as large as 62degrees, in which case p=0.57 λ.

If two beams from a single coherent source, such as a laser overlap, andif environmental vibrations are sufficiently low that the beams do notmove relative to one another by more than about p/2, a recognizablediffraction grating may be recorded in a photographic resist film. Inthe region of overlap of the two beams there is an optical standing wavewhose spatial period is given by the above equation. The standing waveincludes sinusoidally alternating dark and light fringes (i.e., regionsof high and low irradiance). The interference pattern is recorded in thephotosensitive film or resist. The recorded pattern may then be used toform a pattern in an underlying material using well knownphotolithography techniques.

A variety of shapes and sizes of the moth-eye elements 16 may be formed,including relatively blunt pillar-shaped elements 16 a, shown in FIG. 6,and the conical elements 16 shown in FIG. 5. These structures have lowreflectance over large wavelength bands and angular acceptance ranges.The moth-eye like structures can be patterned from sheets of holographicfilm, such as DuPont holographic film, for mass production.

The pattern of elements 16 may form a hologram, in one embodiment of theinvention, which provides the diffusion normally handled by a separatediffuser as well as the effects normally provided by a separatelenticular array. A diffuser expands the viewing area in the verticaldirection and the lenticular array stretches the viewing area along thehorizontal dimensions. By encoding suitable holographic object andreference waves in a holographic medium, using the elements 16 or itssubstrate, the holographic phase plate 14 may replace both the diffuserand lenticular array used in prior structures while providing improvedanti-reflection characteristics.

At the same time, the moth-eye like elements 16 may have relativelysmall feature sizes. For example in one embodiment of the invention,those feature sizes can be on the order of 100 nanometers. This meansthat there will be numerous scattering centers over a small region. Thediffusion effects spatially average over local areas. Due to the spatialaveraging, speckle reduction may be achieved in some embodiments of thepresent invention.

The plate 14 does not adversely effect the outgoing image light. Ineffect it acts as a one way valve allowing the image light to passthrough substantially unaffected while rejecting (and in some caseseffectively eliminating) inbound ambient light, thereby reducing glare.

A display screen may be formed of a support structure and a moth-eyepattern of light absorbing elements. The light absorbing elementsimprove the appearance of the image light outbound of the display whilereducing the reflected light from the display which is viewed by theuser as glare. The screen may be used in both projection displays anddirect view displays and may be formed in a way that the moth-eyestructures provide a holographic diffusing and lenticular array effect.

While the present invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations as fall within thetrue spirit and scope of this present invention.

What is claimed is:
 1. A display screen comprising: a support structure;and holographic phase plate including a pattern of light absorbingelements formed on said structure, said phase plate being defined in thepattern of light absorbing elements, said phase plate to expand theviewing area along the vertical direction and horizontal direction. 2.The screen of claim 1 wherein the elements are conically shaped.
 3. Thescreen of claim 1 wherein the elements are pillar shaped.
 4. The screenof claim 1 wherein the elements are adapted to cause multiplereflections of light hitting one side of the screen and thereby absorb asubstantial percentage of the light.
 5. The screen of claim 1 whereinsaid screen is adapted to act as a screen of a direct view display. 6.The screen of claim 1 wherein said screen is adapted to act as a screenof a front projection display.
 7. The screen of claim 1 wherein saidscreen is adapted to act as a screen of a rear projection display. 8.The screen of claim 1 wherein said support structure includes a Fresnellens.
 9. The screen of claim 1 wherein said elements are in sufficientdensity to reduce the grain size of the display.
 10. The screen of claim1 wherein said elements have features sizes on the order of 100nanometers.
 11. The screen of claim 1 wherein said elements are adaptedto reduce graininess.
 12. The screen of claim 1 wherein said elementsare adapted to reduce speckle.
 13. The screen of claim 1 wherein saidelements are light transmissive.